The objectives of this study are to continue the follow-up of patients entered into prospective Phase II and III therapeutic studies of polycythemia vera. Longer term follow-up to compare mortality and incidence of complications in the three treatment groups (phlebotomy, chlorambucil and 32P) of the PVSG-01 study will permit assessment of optimal therapy. The natural history of polycythemia vera is being characterized and the long term effects of myelosuppressive therapy such as leukemia and cancer determined. Recently, a randomized study had been completed which tested the effect of platelet and anti-aggregating agents plus phlebotomy vs 32-P, the best myelosuppressive treatment. The results were of major significance in that an increased incidence of thrombosis occurred with the anti-aggregating agents suggesting that such agents were not of any benefit. Trials of new agents, such as Hydrea, for treatment of polycythemia and its major symptoms, are being utilized to test its efficacy in management of polycythemia vera. This drug, a non-alkylating myelosuppressive agent, has been highly effective in the treatment of polycythemia vera and its long term effects are being followed, not only with regard to its leukemogenic and oncogenic potential, but for its use as a hemoglobin stimulant of fetal hemoglobin in sickle cell anemia. As of 3/84, 134 patients on the 01 protocol and 104 on the 08 protocol are still being followed. We would like to complete the follow-up on patients on the 01 protocol to the point where 90-95% of the patients have died as well as to continue to accumulate data concerning the long term effects of Hydrea on survival and oncogenesis. To complete this goal, we would like to continue to support a small statistical operation at the current Central Office of the Polycythemia Vera Study Group for the further follow-up and analysis of the patients on these existing protocols of the Polycythemia Vera Study Group.